1. Field of the Invention
The present invention generally relates to image forming apparatuses and, more particularly, to an image forming apparatus, such as a printer, a digital copy machine, a facsimile machine, an optical filing machine or an electronic sorting apparatus, which has a multiple value print engine performing a smoothing operation for removing jaggy portions of an image.
2. Description of the Related Art
Conventionally, there is known an image output device of a digital copy machine provided with a facsimile function. Such an image output device has a multiple value print engine and a frame memory having a multiple value structure so that an image can be represented according to 1 bit/dot, 2 bits/dot or 8 bits/dot by selecting options.
FIG. 1 is a block diagram of a conventional binary image smoothing process circuit. In FIG. 1, the binary image smoothing process circuit comprises a line buffer 100, a template matching process unit 200, a smoothing process unit 300 and a dividing-smoothing process unit 400. The line buffer 100 is a memory for storing binary image data corresponding to a plurality of lines. The line buffer 100 supplies image data, which corresponds to n (dots)×m (lines) matrix pixel data having a center pixel to be processed, to the template matching process unit 200. The template matching process unit 200 compares the n×m matrix pixel data with previously stored template data for smoothing. The result of the comparison is supplied to the smoothing process unit 300. The smoothing process unit 300 transforms the center pixel of the matrix pixel data, which matches the template data for smoothing, to enlarged smoothing pixels consisting of j (dots)×k (dots). The dividing-smoothing process unit 400 applies a dividing-smoothing process to the j×k enlarged smoothing pixels based on a predetermined enlarging ratio so as to convert the j×k enlarged smoothing pixels into the smoothing-processed multiple value data.
Regarding the input binary image data corresponding to a picture or a photograph, which requires gradation representation, the image data is normally supplied to a print engine after a gradation process such as a Dither process or an error diffusion process is applied. On the other hand, the image data corresponding to characters is supplied to the print engine without being subjected to the gradation process since the character image requires sharpness to increase the character recognition rate. The above-mentioned processes are applied to the image data when the image data is subjected to the raster image development in the controller.
The smoothing process circuit shown in FIG. 1 is used to apply the multiple value smoothing process to the binary image data. FIG. 2 is a block diagram of a smoothing process circuit, which processes multiple value image data. The smoothing process circuit shown in FIG. 2 is provided with a line buffer 101 instead of the line buffer 100 shown in FIG. 1. The line buffer 101 stores multiple value image data corresponding to a plurality of lines. Additionally, the smoothing process circuit shown in FIG. 2 is provided with a multiple value smoothing process unit 201, which processes the multiple value image data supplied from the line buffer 101. The multiple value image data is processed by the smoothing process unit 300 and the dividing-smoothing process unit 400 in the same manner as that of the smoothing process circuit shown in FIG. 1.
However, in the above-mentioned structure of the multiple value smoothing circuit, the capacity of the line buffer 101 and the template matching process unit 201 must be increased due to the number of bits representing the multiple values. In order to reduce the capacity of the line buffer and the template matching process unit, there is suggested a multiple value smoothing process circuit shown in FIG. 3.
FIG. 3 is a block diagram of the multiple value smoothing process circuit 600. The multiple value smoothing process circuit 600 has the same structure as the smoothing process circuit shown in FIG. 1 except for a determination block 110 and the selector 500 being added thereto. The determination block 110 is provided before the line buffer 100 so as to determine whether the input multiple value image data is a full intensity pixel. The selector 500 is provided after the dividing-smoothing process unit 400 so as to select one of the input image data and the smoothed data output from the dividing-smoothing process unit 400.
More specifically, when the multiple value image data shown in FIG. 4A including full intensity pixels, 50% intensity pixels and zero intensity pixels is supplied to the multiple value smoothing process circuit 600, the determination block 110 supplies the image data to the line buffer 100 by adding a true value=1 only to each of the full intensity pixels. The template matching process unit 200 applies the template matching process only to the full intensity pixels provided with the true value=1. The thus-processed pixels are subjected to a multiple value smoothing process for binary image by the smoothing process unit 300 and the dividing-smoothing process unit 400. The selector 500 selects and outputs only the thus-processed pixels. Other pixels including the 50% intensity pixels and the zero intensity pixels that are not rendered to be the true value=1 by the determination block 110 are selected by the selector 500, and are output without being subjected to the smoothing process.
The above-mentioned smoothing processes are disclosed in Japanese Laid-Open Patent Applications No. 9-130628 and No. 9-102870.
The multiple value image data may include a character image or a frame image having jaggy portions. Accordingly, if the multiple value smoothing process is applied only to the binary image data, the multiple value image data is not subjected to the smoothing process. As a result there is still a problem in that the jaggy portions remain in the character image or the frame image represented by the multiple value image data.
On the other hand, many multiple value color images include a halftone image part such as a photographic image and a character image part. Thus, if the same smoothing process is applied to the image data corresponding to an entire sheet or page, the smoothing process to be applied to the character image is also applied to the halftone image. That is, if the halftone image part contains a pixel arrangement which matches the template data, the pixel in the pixel arrangement is subjected to the smoothing process to be applied to the character image. This results in deterioration in the degradation of the halftone image part. Additionally, a false contour which does not exist in the original image may be created in the output image. Further, there is a case in which a smoothing operation is preferably applied only to the black character image or a case in which the intensity of the smoothing process is preferably changed according to a color of the image to be processed.